A dispensing system is the assembly at a gasoline station that actually delivers the fuel (e.g. gasoline, diesel fuel, kerosene or alcohol) from a storage tank into the tank of the vehicle in which the fuel is to be used. At most gasoline stations and other locations at which fuel is dispensed, the fuel is stored in an underground storage tank. The dispensing system includes a pump that draws fuel from the storage tank to an above ground elevation so that it will flow into the vehicle fuel tank. In a typical dispensing system, the fuel is pumped from the storage tank, passed through a flow meter and then delivered to the vehicle through a flexible hose. The flow meter performs a volumetric measure of the fuel that is discharged to provide the data needed to ensure that the customer is accurately charged for the amount of fuel delivered. Often this charge data is presented on a display associated with a data processing unit that also forms part of the dispensing system.
The Applicant's Assignee's U.S. Pat. No. 5,884,809, entitled AIR SEPARATING FUEL DISPENSING SYSTEM, issued Mar. 23, 1999, which is incorporated herein by reference, discloses one such fuel dispensing system. This particular fuel dispensing system includes a suction pump that draws the fuel from the underground storage tank. The fuel discharged from the pump is initially forwarded to an air-separating chamber. In this chamber, the fluid discharged from the pump is forced to undergo a cyclonic flow. Forcing the fluid to undergo this type of flow separates air and any vaporized fuel that may be entrained in the liquid-state fuel stream. The air-removed fuel stream is then forwarded downstream to the metering device and, once metered, to the hose for delivery to the vehicle.
The air, and any fuel entrained therein, extracted from the fuel stream is forwarded to an air elimination chamber. This chamber serves as an enclosed space in which the fuel that is contained in the air stream passively separates from the air. This fuel is returned to the fuel stream upstream of the pump. The air is exhausted into the ambient environment.
The above-described fuel dispensing system also has a sub-assembly that detects the quantity of air in the fuel stream discharged from the pump. This sub-assembly is necessary because sometimes, the quantity of entrained air is so large, the air separator is not able reduce the air in the pumped fuel stream down to the essentially zero-level preferred by individuals purchasing fuel. The air-detecting sub-system in the '809 Patent operates by monitoring the differential pressure across the air separation chamber. A drop in this pressure difference is recognized as an indication that there is an appreciable rise in the quantity of air present in the fuel stream discharged from the pump. This differential pressure measurement is made with either a fluid-set valve or an electronic transducer. If a pre-determined drop in the differential pressure is detected, a fuel shut-off valve connected to the air-monitoring sub-system is set closed. Thus, this sub-system continually monitors the quantity of air in the fuel stream and, when the quantity of air reaches unacceptable levels, prevents the dispensing of the fuel. The above-described air-detecting sub-system works reasonably well for evaluating the quantity of air and vaporized fuel in fuel stream that is discharged from a pump. However, it requires one to install two fluid-carrying conduits to the air separation chamber of the dispensing system in which this sub-system is installed. Having to provide these conduits and the associated differential pressure sensor can significantly add to the cost of the associated dispensing system.
There have been some attempts to provide air detecting sub-systems for use with a dispensing system that monitor only a single parameter. One such system operates by monitoring the density of the gaseous state fluids removed by the air separator. Another system operates by monitoring the force of a jet of gaseous state fluid that is discharged from the air separator. A disadvantage of these systems is that they both measure multi-state fluid streams. Because these streams either are formed from two different types of fluids, or fluids in both the gas and liquid state, the measurement of these streams may sometimes not accurately indicate the volume of air that is present in the fuel stream discharged from the pump.